History Series / Signal

* MyGreatestFailure: The four cold cases that the Cold Case squad is tasked with solving are all cases that Lee Jae-han considered his greatest failures either because he could not catch the culprit right away or because he was he was thwarted by other factors.

* BolivianArmyEnding

* ChekhovsGun: Lee Jae-han's notebook.

* RippleEffectProofMemory: Lt. Park's memory remains unaffected when his communication into the past changes the past, and Jae-han saves a victim of a serial killer. Park is the only only one who remembers that in the past (that is, in the original, alternate past) that particular woman was in fact murdered.

* HeartbrokenBadass: Jae-han, in a manner of speaking. [[spoiler: Won-kyung's death spurs Jae-han into become a detective who will not rest until victims receive justice.]]

* PoliceAreUseless: Lt. Park's reason for his grudge against them.

* RooftopConfrontation: The first serial killer arc ends in episode 4 with a rooftop confrontation between Lee Jin-hyung, the killer; Lee Chun-goo, his father, and Detective Lee Jae-han. Jin-hyung winds up falling off the roof which is why he's a paraplegic in 2015.

* AnachronicOrder: Due to the nature of the story with Jae-han and Hae-young talking to each other over the decades via the PortalToThePast walkie-talkie. The series jumps back and forth between the present day of 2015 (where Hae-young and Soo-hyun are investigating cold cases), 1989 and Jae-han's involvement in the South Gyeonggi serial killer case, 1995 and Jae-han's initial friendship with Soo-hyun, and 2000 and the kidnap-murder of the the little girl.

* AnachronicOrder: Due to the nature of the story with Jae-han and Hae-young talking to each other over the decades via the PortalIntoThePast walkie-talkie. The series jumps back and forth between the present day of 2015 (where Hae-young and Soo-hyun are investigating cold cases), 1989 and Jae-han's involvement in the South Gyeonggi serial killer case, 1995 and Jae-han's initial friendship with Soo-hyun, and 2000 and the kidnap-murder of the the little girl.

* TheDogBitesBack: Sick of being his boss' whipping boy and haunted by killing Lee Jae-han, Ahn Chi-soo hides that Park Hae-young is Sun-woo's little brother and offers to tell him everything.
* GlassesPull: The detective in charge of the 1989 investigation into the serial killings does this in episode 3, when Hae-young comes to ask. He then screams that everything that went wrong with that case was Jae-han's fault. (We later learn that Jae-han nabbed the wrong guy, who had a seizure and died after the cops slapped him around.)

* CallBack: The TimePassesMontage of Hae-young's troubled youth in episode 1 shows him getting into a fight in a pool hall. Episode 12 reveals that he was in the pool hall confronting one of Sun-woo's classmates, the one who gave false witness and told the cops that Sun-woo took the victim of the gang rape on a bus.

* PortalToThePast: Much like ''{{Film/Frequency}}'', it can only transmit sound.

* RippleEffectIndicator: Both the notes in Hae-young's notebook, and the notes he's written on a whiteboard in the squad room, change when his communication into the past leads to Jae-han saving the eighth victim of the serial killer in 1989. As Hae-young stares in astonishment, the notes on the eighth victim change from murder to attempted murder.

* MyGreatestFailure: The four cold cases that the Cold Case squad is tasked with solving are all cases that [[spoiler: Lee Jae Han considered his greatest failures either because he could not catch the culprit right away or because he was he was thwarted by other factors.]]

* ButterflyOfDoom: Both Jae-han and Hae Young learn the hard way that actions have consequences, especially when it comes to the past.

** [[spoiler:In the first arc, the culprit of the kidnapping case is arrested for the murder of a man who knew of her crime, but the statute of limitations for the abducted girl's death has passed and can't be prosecuted, much to the despair of the mother. These events lead to a backlash against the statute of limitations, causing the government to repeal the policy on unsolved murders, leading the formation of the Cold Case Squad.]]

* OneWordTitle

* PosthumousSibling: [[spoiler: Park Sun-Woo. The mystery of his death plays a big role in the second half of the drama.]]

* TheProfiler: Park Hae-young himself. While he is good at deductions, he also stresses that it is not an exact science.
* TheLostLenore: Won-Kyung for Jae Han. He's never really able to get over it and the fact that the couldn't save her in time forever remains a source of grief.

A Japanese version based on the show has aired on Fuji TV under the [[Series/Signal2018 same title]].

A Japanese version based on the show is currently airing on Fuji TV under the [[Series/Signal2018 same title]].

* DeadAllAlong: [[spoiler: When he first talked to Detective Lee, Lt. Park searched for any members of the police with his name, only finding people with the same name and a file that says he's missing. After realizing the walkie-talkie communicates with the past, he does some digging and later finds his remains underneath a house. It is later revealed that Detective Lee was murdered by a crooked cop for having evidence of Lt. Park's brother being murdered by his boss.]]

* NearDeathExperience: All the three leads have near death experiences during the course of the drama.[[spoiler: Cha Soo-hyun almost becomes the victim of a serial killer. However, she is able to escape in time and is luckily found by Jae-han. Park Hae-young takes a bullet for Cha Soo-hyun and comes close to losing his life. Lee Jae-han survives an attempt on his life by Kim Sung-bum, Kim Bum-joo and Ahn Chi-soo. Luckily for him, he's able to call his team for back-up and they arrive in the nick of time.]]